1. Field of Invention
The invention relates to a coordinate reading device that reads a coordinate of a position of a coordinate input device, in accordance with a signal generated on a plurality of coils laid in a coordinate input sheet by an alternating magnetic field generated from the coordinate input device.
2. Description of Related Art
FIG. 35 shows a structure of a conventional coordinate reading device. (Japanese Laid-Open Patent Publication No. 5-165560).
The coordinate reading device shown in FIG. 35 includes a tablet (coordinate input sheet) 91, a scanning circuit 92, and a detection circuit 90. The tablet 91 is provided with a plurality of X-sense coils X1 to Xm for detecting X-coordinates of a stylus (coordinate input device) 100 and Y-sense coils Y1 to Yn for detecting Y-coordinates of the stylus 100. The scanning circuit 92 serially scans the sense coils X1 to Xm and Y1 to Yn in the tablet 91. The detection circuit 90 calculates the X- and Y-coordinates of the stylus 100 by detecting induction signals generated at the sense coils X1 to Xm and Y1 to Yn.
The stylus 100 includes a coil 101 that generates an alternating magnetic field. When the stylus 100 contacts the tablet 91, some sense coils located near the stylus 100 generate induction signals 97 due to magnetic coupling with the alternating magnetic field from the coil 101. Selection signals 98, that serially specify the sense coils X1 to Xm and Y1 to Yn to detect the respective introduction signals 97 generated at the sense coils X1 to Xm and Y1 to Yn, are inputted into the scanning circuit 92. The induction signals 97 are inputted into the detection circuit 90. The inputted induction signals 97 are amplified at an amplifier 93 where the amplitude is detected at an amplitude detection circuit 94. Next, an A/D conversion circuit 95 converts the measured amplitude into digital values, and outputs the digital values to a CPU 96. The CPU 96 calculates a positional coordinate of the stylus 100 based on the inputted digital values from the A/D conversion circuit 95. For example, the CPU 96 may refer to a coordinate table that stores data indicative of a relationship between a plurality of digital values and coordinate positions. The CPU 96 selects a positional coordinate that corresponds to the digital values presently inputted from the A/D conversion circuit 95.
When the stylus 100 is tilted against a coordinate input plane of the tablet 91, a distribution of the alternating magnetic field generated from the coil 101 is changed. Therefore, a voltage generated on the sense coils is changed.
However, the digital values in the coordinate table used in the conventional coordinate reading device are measured in advance, based on the voltage generated at the sense coils due to the magnetic coupling with the alternating magnetic field from the coil 101 when a center axis of the stylus 100 is perpendicular to the coordinate input plane (that is, when the center axis of the stylus 100 is not tilted against the coordinate input plane). Accordingly, when the stylus 100 is tilted against the coordinate input plane of the tablet 91, an error occurs in the coordinate by selecting the coordinate of the digital value corresponding to the voltage generated at the sense coils.
Further, when the output level of the alternating magnetic value is reduced due to exhaustion of a battery in the stylus 100, induction signals generated at the sense coils due to the alternating magnetic field having the reduced output level is detected and coordinates corresponding to an amplitude of the induction signals are selected, thereby reducing reading accuracy of the coordinates.
The invention provides a coordinate reading device that can precisely read coordinates of a coordinate input device even when the coordinate input device is tilted against a coordinate input area or when an output level of an alternating magnetic field generated from the coordinate input device is reduced.
In various embodiments, the coordinate reading device includes a coordinate input sheet that has a coordinate input area for inputting a coordinate by a coordinate input device generating an alternating magnetic field, and a plurality of loop coils laid under the coordinate input area; a coil detecting device that detects a coil that generates a maximum signal level and a coil that generates a first signal level that is a larger signal level generated from one of coils adjacent to the coil generating the maximum signal level, in accordance with a strength of the alternating magnetic field generated from the coordinate input device; a first storage device that stores a first relationship between the maximum signal level and the first signal level and a second relationship between the first relationship and a distance between a predetermined position and the coordinate input device on the coordinate input area; a reading controller that reads a distance, corresponding to the first relationship obtained based on the detected maximum signal level and the first signal level, from the first storage device, and reads a positional coordinate of the coordinate input device on the coordinate input area based on the read distance; a signal level detecting device that detects a second signal level in which two signal levels detected from coils adjacent to each other of the plurality of coils are substantially equal to each other, in accordance with the strength of the alternating magnetic field; a signal level calculating device that calculates a third relationship between a third signal level and a fourth signal level which are generated from at least two coils adjacent to the outside of the adjacent coils generating the second signal level detected by the signal level detecting device; a second storage device that stores relationships between the second signal level and the third relationship and a correction; and a correcting device that corrects the second relationship stored in the first storage device by reading the correction, corresponding to the second signal level detected by the signal level detecting device and the third relationship calculated by the signal level calculating device, from the second storage device.
The signal level detecting device detects a second signal level in which two signal levels detected from coils adjacent to each other of the plurality of coils are substantially equal to each other, in accordance with the strength of the alternating magnetic field.
That is, in the coordinate reading device that detects coordinates of the coordinate input device based on a difference between two signal levels detected from the adjacent coils of the plurality of coils, when the coordinate input device exists in a middle of two coils adjacent to each other, the difference of the two signal levels generated on the two coils is zero, that is, the two signal levels indicate the same value. This relationship is also established when a strength of the alternating magnetic field generated from the coordinate input device becomes weak or when the coordinate input device is tilted with respect to the coordinate input area.
Therefore, if the two signal levels are detected when the relationship is established, the strength of the alternating magnetic field can be obtained.
The signal level calculating device calculates a third relationship between a third signal level and a fourth signal level which are generated from at least two coils adjacent to the outside of the adjacent coils generating the second signal level detected the second signal level detecting device.
That is, when the coordinate input device is perpendicular to the coordinate input area (when the coordinate input device is not tilted against the coordinate input area), a relationship that the third signal level and the fourth signal level, which are generated from at least two coils adjacent to the outside of the adjacent coils generating the second signal level detected the second signal level detecting device, are the same, is established. However, when the coordinate input device is tilted against the coordinate input area, the third signal level and the fourth signal level indicate a different value, so that the relationship described above cannot be established.
The second storage device stores the relationships between the second signal level and the correction and between the third relationship and the correction.
That is, as the second signal level changes, the third relationship between the third signal level and the fourth signal level changes. Therefore, the second storage device stores the relationships between the second signal level and the correction and between the third relationship and the correction.
The correction device corrects the second relationship stored in the first storage device by reading the correction, corresponding to the second signal level detected by the signal level detecting device and the third relationship calculated by the signal level calculating device, from the second storage device.
That is, when the second signal level is detected, the correction device calculates the third relationship corresponding to the second signal level by the signal level calculating device, reads the correction corresponding to the calculated third relationship from the second storage device, and corrects the second relationship stored in the first storage device based on the read correction.
Thus, every time the second signal level is detected, the second relationship stored in the first storage device can be corrected. Accordingly, the coordinate reading device can be provided that can precisely read coordinates of a position of the coordinate input device even when the coordinate input device is tilted against the coordinate input area or when an output level of the alternating magnetic field generated from the coordinate input device is reduced.
According to one exemplary aspect of the invention, the correction stored in the second storage device may be an amount of linear changes in the first relationship and the distance which are obtained in advance by measurement when a center axis of the coordinate input device is tilted against the coordinate input area from a state where the coordinate input device is perpendicular to the coordinate input area.
That is, the second relationship for obtaining the coordinates of the coordinate input device is a relationship between the first relationship and the distance between the predetermined position and the coordinate input device on the coordinate input area. The second relationship can be expressed by a substantially linear relationship. Thus, it can be regarded that the second relationship when the center axis of the coordinate input device is tilted from the state where the coordinate input device is perpendicular to the coordinate input area substantially linearly changes.
Accordingly, a linear correction of the coordinate can be performed by storing the amount of linear changes obtained in advance by measurement, as a correction, in the second storage device. Therefore, the coordinate reading device can precisely read the coordinates of the coordinate input device even when the coordinate input device is tilted against the coordinate input area or when the output level of the alternating magnetic field generated from the coordinated input device is reduced.
According to another exemplary aspect of the invention, the correction stored in the second storage device may be an expression that establishes the relationship between the first relationship and the distance when the center axis of the coordinate input device is tilted against the coordinate input area from a state where the coordinate input device is perpendicular to the coordinate input area, by the second signal level and the third relationship obtained in advance by the measurement.
That is, the second relationship for obtaining the coordinates of the coordinate input device, for example, can be expressed by an expression formed by the second signal level and the third relationship.
Therefore, a correction of the coordinate can be performed by storing the expression when the center axis of the coordinate input device is tilted against the coordinate input area from a state where the coordinate input device is perpendicular to the coordinate input area, as a correction, in the second storage device. Therefore, the coordinate reading device can precisely read the coordinates of the coordinate input device even when the coordinate input device is tilted against the coordinate input area or when the output level of the alternating magnetic field generated from the coordinated input device is reduced.
According to another exemplary aspect of the invention, the coordinate input device may include a power supply therein, and the second storage device may store relationships between the second signal level, the third relationship and voltage of the power supply and the correction when the center axis of the coordinate input device is tilted against the coordinate input area from a state where the coordinate input device is perpendicular to the coordinate input area. The coordinate reading device may further include a warning device that warns of exhaustion of the power supply when the voltage of the power supply, which is read from the second storage device and corresponding to the second signal level detected by the second signal level detecting device and the third relationship calculated by the signal level calculating device, is lower than a predetermined valve.
That is, as the voltage of the power supply built in the coordinate input device decrease, the strength of the alternating magnetic field generated from the coordinate input device becomes weak. Thus, the second signal level decreases and the third relationship changes.
The second storage device stores the second signal level and relationships between the third relationship and voltage of the power supply and between the third relationship and the correction when the center axis of the coordinate input device is tilted against the coordinate input area from a state where the coordinate input device is perpendicular to the coordinate input area. Accordingly, when the second signal is detected, the warning device can warn of exhaustion of the power supply when the voltage of the power supply is lower than the predetermined voltage, by reading the second signal level detected by the second signal level detecting device and the voltage of the power supply corresponding to the third relationship calculated by the signal level calculating device.